The invention concerns a mass flowmeter for measuring by means of the Coriolis principle both mass and density of a fluid flowing in pipes and establishing oscillations in these pipes, said mass flowmeter comprising an inlet and an outlet for the fluid, between which is arranged at least one measuring pipe, said measuring pipe being provided with sensors for the detection of movements in the measuring pipe in the form of fluid flow through the measuring pipe, said sensors consisting of at least one magnet fixed to the measuring pipe and at least one coil fixed to a coil supporting plate.
U.S. Pat. No. 5,349,872 describes a device of this kind. The magnets are arranged on a first measuring pipe and a second measuring pipe, and the coils are arranged on a frame for the mass flowmeter. Various ways of arranging the magnets and coils and various types of coils are described in the document. Measurements made with this device, which are described in the document, depend on the way the magnets are arranged on the measuring pipes and the type of coils used. Generally, the signal size will depend on the strength of the magnet and the size of the coil, so that a strong magnet will only demand a small coil and vice versa, to obtain a given signal size. The embodiments shown in the document are all realised with magnets fixed to the measuring pipes to reduce the mass of the measuring pipes and to facilitate the mounting of the cables leading to the coils. The mass of the magnets is normally smaller than the mass of the coils.
However, all the device embodiments shown in the document mentioned above imply one considerable disadvantage. The measurement accuracy depends among others on the linearity of the movement sensor described. The linearity again depends on the construction of the coil and the magnet. The coil in the embodiments shown are all made with one single winding. This implies that to get a linear signal, the magnet must be made so that it creates a homogenous field in the spot where the magnet influences the coil. This increases the complexity of the magnet, the weight increases, and the demands on positioning of magnet and coil are increasing. Further, the device can only make accurate measurements at a limited oscillation of the measuring pipes, as the linearity of the voltage induced in the coils only extends over a limited deflection of the measuring pipes.